It is known that for drawing optical fibres, the material composing the preform should be heated to temperatures above 2000.degree. C. (e.g. temperatures of about 2200.degree. C.). These temperatures are generally attained by resistance furnaces or by induction furnaces operating at radiofrequency.
Induction furnaces are preferred because resistance furnaces (and particularly furnaces with a resistive heater made of graphite) need an inert gas to avoid combustion of the resistive heater, are liable to pollute the reaction area because of the reactivity of the heater with the environment, and have reduced life because of the resistive heater wear.
Generally, in induction furnaces the heater is a cylinder of ZrO.sub.2 stabilized with Y.sub.2 O.sub.3. Such a composition is inert, so that it allows operation in air, and has a low thermal conductivity, whereby it ensures a greater stability in the heating area, with a more suitable thermal gradient in axial direction. Moreover the heater is not subjected to wear, hence the thermal conditions are stable and reproducible, so that a good uniformity of the fibers manufactured is achieved.
Especially for the heaters of induction furnaces the problem arises of high energy particle emission towards the converging bottom portion of the preform. That emission may occur due to surface unevennesses which cause localized temperature increases, with sublimation of the heater material, or localized concentration of mechanical stresses.
The emitted particles impinge on the preform surface and readily penetrate it due to its low viscosity. Discontinuities in fiber surface therefore arise, which can constitute localized fracture centers. This reduces the mechanical performance of the fiber and in particular reduces tensile strength. This may result in the need to reject the fiber because it does not pass the first tensile strength test (the so called screening test, in which the fiber is subjected to tensile stresses of 5 to 15N), with consequent economical loss, or may pose the risk of installing fibres which are defective even if they have passed the tests.
The literature has not disclosed until now any solution to this problem. Since the problem depends on the nature of the heater surface, the more obvious solution would seem to be surfacing the heater during manufacture so as to render the surface itself regular and compact. Yet the high temperatures to be attained to modify the surface (zirconia has a melting point of 2715.degree. C.) make it substantially impossible or in any event not advantageous from the economic standpoint to intervene during heater manufacture.